1. Field of the Invention
The present invention relates to a cellular communication system. More particularly, the present invention relates to a system and a method for allocating frequency resources based on multiple frequency reuse factors in a cellular communication system using an orthogonal frequency division multiple access (OFDMA) scheme.
2. Description of the Related Art
In general, in a cellular communication system, the same frequency resources can be used in two areas even when they are spaced apart from each other to achieve efficient and effective use of limited frequency resources. The concept of frequency reuse will be described with reference to FIG. 1, which is a schematic view illustrating the concept of frequency reuse in a conventional cellular communication system.
Referring to FIG. 1, frequency resource F1 used in a first cell 100 having a radius R can be used in a second cell 150 having a radius R, which is spaced from the first cell 100 by a distance D. This is called “frequency reuse”.
A frequency reuse factor K is obtained when the same frequency resource, that is, the same frequency band, is reused in K cell units. As the frequency reuse factor increases, a distance D between frequency reuse cells using the same frequency resource also increases. In addition, a wave is attenuated in proportion to a propagation distance, so that interference from using the same frequency resource is reduced as the frequency reuse factor is increased. The amount of frequencies available in one cell can be obtained by dividing the whole frequency band by the frequency reuse factor K, so efficiency of the whole system may is adversely affected as the frequency reuse factor increases.
Frequency resource allocation according to the frequency reuse factor K will be described with reference to FIGS. 2A to 2F. FIG. 2A is a schematic view illustrating frequency resource allocation when the frequency reuse factor K is 3(K=3). Referring to FIG. 2A, if the frequency reuse factor K is 3,
  1  3of the whole frequency band is allocated to each of the three cells. FIG. 2B is a schematic view illustrating frequency resource allocation when the frequency reuse factor K is 4(K=4). As shown in FIG. 2B, if the frequency reuse factor K is 4,
  1  4of the whole frequency band is allocated to each of the four cells.
FIG. 2C is a schematic view illustrating frequency resource allocation when the frequency reuse factor K is 7(K=7). When the frequency reuse factor K is 7,
  1  7of the whole frequency band is allocated to each of the seven cells.
FIG. 2D is a schematic view illustrating frequency resource allocation when the frequency reuse factor K is
      3    9    ⁢            (              K        =                  3          9                    )        .  In this case,
  3  9of the whole frequency band is allocated to each three cell unit of the total nine cells, respectively, so that the frequency reuse factor
  K  ⁢      3    9  is applied to each of nine cells.
FIG. 2E is a schematic view illustrating frequency resource allocation when the frequency reuse factor K is
      4    12    ⁢            (              K        =                  4          12                    )        .  The whole frequency band is allocated to each three cell unit of the total twelve cells, so that the frequency reuse factor
  K  ⁢      4    12  is applied to each of twelve cells.
FIG. 2F is a schematic view illustrating frequency resource allocation when the frequency reuse factor K is
      7    21    ⁢            (              K        =                  7          21                    )        .  As shown in FIG. 2F, if the frequency reuse factor K is
      7    21    ,      1    7  of the whole frequency band is allocated to each three cell unit of the twenty-one cells so that the frequency reuse factor K
  7  21is applied to each of the twenty-one cells.
In analog cellular communication system, a minimum signal to noise ratio (SNR) is required for making a wireless voice communication circuit. To satisfy the SNR, a minimum distance between cells is defined. The frequency reuse factor is also defined based on the SNR.
In digital cellular communication system, a minimum SNR has various values depending on an error correction coding rate applied to the wireless circuit, modulation scheme, and transmission scheme. In particular, a code division multiple access (CDMA) communication system applies a frequency reuse factor of “1” to all cells by taking the minimum SNR, system capacity, and network design into consideration. Since the CDMA communication system applies the same frequency band to all cells, a code spreading/de-spreading process discriminates the cells from each other. In this manner, interference of adjacent cells is averaged so that data of a present service cell can be discriminated from data of other cells.
The frequency reuse factor is an important factor in a radio packet cellular communication system using an orthogonal frequency division multiple access (OFDMA) scheme. As discussed above, if the frequency reuse factor K=1, system capacity improves and network design is easier. Hereinafter, a carrier to interference and noise ratio (CINR) of a downlink signal in a cellular communication system with the frequency reuse factor of 1 is described with reference to FIG. 3.
FIG. 3 is a schematic view illustrating the CINR of a downlink in a cellular communication system employing the frequency reuse factor of 1. As shown in FIG. 3, in a cell center region 301 adjacent to a base station (BS), intensity of a downlink signal, that is, the CINR is not influenced by intensity of an interference signal having the same frequency band from adjacent cells, so a relatively high CINR is present. However, a cell boundary region 303 spaced from the BS is significantly influenced by the interference signal having the same frequency band from adjacent cells, so a relatively low CINR is present.
When subscriber stations (SSs) are located in the cell boundary region 303, if the cellular communication system provides a low error correction coding rate and a low modulation scheme, frequency efficiency of the SSs in the cell boundary region 303 may degrade even though the SSs can normally receive packet data from the BS.
To solve the above problem, the frequency reuse factor K is set to K>1. Even if the frequency reuse factor K is set to K>1, the signal may be attenuated in proportion to the propagation distance of the wave, so the CINR of the downlink decreases in a direction of the cell boundary region 303. However, since the interference component is very small, the CINR of the downlink is relatively high if the frequency reuse factor K is set to K>1 as compared with the CINR of the downlink when the frequency reuse factor is equal to 1. This will be described in detail with reference to FIG. 4.
FIG. 4 is a graph illustrating the relationship between the CINR and a distance from the BS when a frequency reuse factor is 1 (K=1) and greater than 1 (K>1) are applied to the cellular communication system. As shown in FIG. 4, as the frequency reuse factor increases, frequency efficiency in the cell boundary region can improve. However, since each cell uses 1/K of the whole frequency band, capacity of the whole system is reduced as compared with that of the system employing the frequency reuse factor of 1.